Anisotropic etch method

ABSTRACT

A method to anisotropically etch an oxide/silicide/poly sandwich structure on a silicon wafer substrate in situ, that is, using a single parallel plate plasma reactor chamber and a single inert cathode, with a variable gap between cathode and anode. This method has an oxide etch step and a silicide/poly etch step. The fully etched sandwich structure has a vertical profile at or near 90° from horizontal, with no bowing or notching.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/571,523,filed May 16, 2000, now U.S. Pat. No. 6,461,976, issued Oct. 8, 2002,which is a continuation of application Ser. No. 08/909,229, led Aug. 11,1997, now U.S. Pat. No. 6,133,156, issued Oct. 17, 2000, which is adivisional of application Ser. No. 08/603,573, filed Feb. 20, 1996, nowU.S. Pat. No. 5,958,801 is issued Sep. 28, 1999, which is a continuationof application Ser. No. 08/194,134, led Feb. 8, 1994, abandoned, whichis a divisional of application Ser. No. 07/574,578, led Aug. 27, 1990,now U.S. Pat. No. 5,201,993, issued Apr. 13, 1993, which is acontinuation of application Ser. No. 07/382,403, filed Jul. 20, 1989,now U.S. Pat. No. 5,271,799, issued Dec. 21, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to etching methods used in the fabricationof integrated electronic circuits on a semiconductor substrate such assilicon, particularly a single-chamber/single-cathode (in situ) methodof anisotropically plasma etching a sandwich structure of an oxide,tungsten silicide, and polycrystalline silicon, or equivalent structure.

An electronic circuit is chemically and physically integrated into asubstrate such as a silicon wafer by patterning regions in thesubstrate, and by patterning layers on the substrate. These regions andlayers can be conductive, for conductor and resistor fabrication, orinsulative, for insulator and capacitor fabrication. They can also be ofdiffering conductivity types, which is essential for transistor anddiode fabrication. Degrees of resistance, capacitance, and conductivityare controllable, as are the physical dimensions and locations of thepatterned regions and layers, making circuit integration possible.Fabrication can be quite complex and time consuming, and thereforeexpensive. It is thus a continuing quest of those in the semiconductorfabrication business to reduce fabrication times and costs of suchdevices in order to increase profits. Any simplified processing step orcombination of processes at a single step becomes a competitiveadvantage.

2. State of the Art

A situation where a process simplification is desirable is in theanisotropic etch of a layer of oxide on a layer of silicide on a layerof poly (also called an oxide/silicide/poly sandwich structure). In thisdisclosure, “oxide” denotes an oxide of silicon, “silicide” is short fortungsten silicide, and other commonly known silicides such as tantalumsilicide, molybdenum silicide, and titanium silicide, and “poly” isshoptalk for polycrystalline silicon. “Polycide” denotes asilicide-over-poly combination. Oxide is an insulator with dielectricproperties. Poly is resistive in nature, but is made less resistive whendoped with an element having less or more than four valence electrons,or when layered with conductive silicide.

An oxide/silicide/poly sandwich structure presents a difficult etchingtask, particularly with an additional mask layer of photoresist(“resist”), which must be the case if patterning is desired. Thedifficulty is due to the distinct differences in the way oxide andpolycide are etched, particularly with resist still present on top ofthe structure.

Both oxide and polycide can be etched using a parallel plate plasmareactor. However, an oxide is typically etched in fluorine-deficientfluorocarbon-based plasmas, whereas silicide and poly can be etched influorine- or chlorine-based discharges. Reactor cathode materials mayalso differ: for oxide etch, an erodible cathode such as graphite orsilicon is often used to provide a source of carbon or silicon for etchselectivity, whereas for polycide etch, an inert cathode is preferred,especially when utilizing chlorine gas (Cl₂) for selectivity. If asingle-chamber process were attempted using conventional art to etch anoxide/silicide/poly sandwich structure, the erodible cathode requiredfor oxide etch would be destroyed by the chlorine required for polycideetch. Using conventional methods, the two steps are incompatible.

Oxide etch in general is fairly well understood given a universal needfor a vertical profile. This vertical profile is realized primarily byion induced reaction with the oxide, coupled with normal incidence ofthe ions on the oxide surface. The amount and energy of these ions areprimarily controlled by the reactor's rf power and gap. Generally, afluorocarbon-based gas mixture is introduced at a low pressure into theetch chamber. The exact gas composition is chosen, and an erodiblecathode is used to scavenge excessive fluorine radicals so that thefluorine-to-carbon ratio is near, but not beyond, the so-calledpolymerization point. Under these conditions, when a plasma is ignited,the fluorocarbons are dissociated and release fluorine radicals andCF_(x) species. Although fluorine radicals etch oxide, they do so veryslowly: the primary etchant for oxide is considered to be the CF_(x)species. Some of these species diffuse to the oxide surface where, withthe assistance of ion bombardment, they react with the oxide and releasevolatile byproducts SiF₄, CO, and CO₂. In addition, some of the CF_(x)species react with each other to form fluorocarbon polymers. Polymerthat forms on horizontal surfaces is removed by vertical ionbombardment. Polymer that forms on vertical sidewalls is notsignificantly degraded by the bombardment, and actually serves a usefulpurpose by protecting the sidewalls from attack by the etchant species.This sidewall protection enables the achievement of vertical profiles,adjustable by varying the fluorine-to-carbon ratio. As the cathode iseroded, the quantity of available fluorine radicals is reduced.Therefore, a polymer-producing gas such as CHF₃ is balanced against afluorine-producing gas such as CF₄ to provide proper selectivity, withassistance to sidewall protection.

Two methods are presently used to etch an oxide/silicide/poly sandwichstructure. Both methods use separate reactors: one for oxide etch andone for polycide etch. The first method involves etching the oxide in anoxide etch reactor, using an erodible cathode. After oxide etch, theresist is removed from the wafer. Silicide and poly are then etched in apoly etch reactor, using an inert cathode. Both etches are anisotropic.

The second method uses the same principles as the first, except thatthere are two reactors in one machine. The two reactors are configuredas separate oxide and polycide reactors having a common vacuum transferarea, so that a wafer can be transferred in a vacuum from the oxidereactor to the polycide reactor, thus minimizing additional handling.The resist is generally not removed prior to polycide etch in thismethod. This is sometimes referred to as “in situ” since the wafersnever leave the vacuum of one machine. However, they are etched in twodifferent etch chambers and are not truly in situ in the sense of thisdisclosure.

It would be of great advantage to etch oxide and polycide truly “insitu,” that is, in one reactor chamber, with a single cathode.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a method ofanisotropically etching an oxide/silicide/poly sandwich structure insitu. Other objects of the invention are a fast processing time, animproved process yield and cleanliness.

In summary, the inventive process is as follows. A wafer having thesandwich structure described above, coated with a mask layer of resist,is transferred into the chamber of a parallel plate plasma reactor,having an anodized aluminum cathode and a variable gap, for two steps:oxide etch and polycide etch. In the oxide etch step, oxide notprotected by resist is exposed to a plasma of about 1.9 W/cm² powerdensity at a 0.48 cm gap, in a 2.3 torr atmosphere of 50 sccm C₂F₆, 100sccm He, 40 sccm CF₄, and 32 sccm CHF₃. Immediately after the oxide etchstep, in the same chamber and using the same cathode, silicide and polylayers are etched in a plasma of about 0.57 W/cm² at a 1.0 cm gap in a0.325 torr atmosphere of 90 sccm Cl₂ and 70 sccm He. The finishedstructure has a vertical profile at or near 90° from horizontal, with nobowing or notching. The entire inventive process takes about 3 minutes.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a cross-sectioned oxide/silicide/poly sandwich structurewith a patterned resist mask layer, prior to the inventive etch.

FIG. 2 shows a cross-section of said structure after oxide etch.

FIG. 3 shows a cross-section of said structure after polycide etch.

DETAILED DESCRIPTION OF THE-INVENTION

As illustrated in FIG. 1, a photoresist mask layer 10 is aligned anddeveloped on a sandwich structure of oxide 11, silicide 12, and poly 13on gate oxide 14 of a silicon wafer substrate 15. Fabrication anddamasking of this structure are done by methods well known to thoseskilled in semiconductor design and processing, and hence are not fullydisclosed herein. The preferred embodiment of the inventive method iswell suited to etch a 3,000 angstrom layer 11 of TEOS oxide (an oxide ofsilicon, derived from tetraethylorthosilicate) on 1,200 angstroms oftungsten silicide 12 on 3,000 angstroms of poly 13.

The wafer having the masked structure is transferred into the chamber ofa Lam 790 parallel plate plasma reactor, having an anodized aluminumcathode, a variable gap, and a 13.56 MHz rf power plasma generator foran inventive process having two steps: oxide etch and polycide etch. Inthe oxide etch step, oxide 11 not protected by photoresist mask layer 10is exposed to a plasma of about 1.9 W/cm² power density at a 0.48 cmgap, in a 2.3 torr atmosphere of 50 sccm C₂F₆, 100 sccm He, 40 sccm CF₄,and 32 sccm CHF₃. In this disclosure, “sccm” denotes standard cubiccentimeters per minute, and “gap” refers to the distance between plasmaelectrodes, one of which supports the wafer. After the oxide etch step,which takes under a minute, the structure appears as shown in FIG. 2.

Immediately after the oxide etch step, in the same chamber and using thesame cathode, silicide and poly layers 12 and 13 are etched in a plasmaof about 0.57 W/cm² at a 1.0 cm gap in a 0.325 torr atmosphere of 90sccm Cl₂ and 70 sccm He. This etch takes a little over 2 minutes, withthe entire inventive process taking about 3 minutes. The finishedstructure appears as shown in FIG. 3, with a profile at or near 90° fromhorizontal, with no bowing or notching.

Details of the oxide etch step are now provided. Although preferredparameter values are stated above, plasma power density can range withinabout 0.18-4.0 W/cm², the gap can vary within about 0.3-0.6 cm,0.38-0.52 cm being the preferred range, and the pressure can rangewithin about 1.8-3.0 torr, although 2.2-2.4 torr is preferred. Gasquantities may vary, as long as at least about 5 sccm He is provided.Providing more CF₄ than CHF₃ makes a cleaner process, but this ratio canbe varied if desired.

The inventive process uses a non-erodible anodized aluminum cathode,which increases the amount of available fluorine radicals. According toconventional thought, in order to maintain the same oxide-to-polycideselectivity as the prior art, the ratio of CF₄ to CHF₃ must be decreasedto minimize fluorine radicals. It was found that this approach does notprovide adequate selectivity without an excessive and quick buildup ofpolymer. This was solved by adding C₂F₆ to the chamber atmosphere as thepredominant gas, which provides more CF_(x) species and relatively fewfluorine radicals, resulting in acceptable selectivity without excessivepolymer buildup. C₂F₆ also resolves a “micromasking” problem, in whichareas of underlying polycide were not being etched. Although the causeis unclear, it is speculated that the CF_(x) species reacted with thetungsten silicide, forming a polymer layer which interfered withsubsequent polycide etching. C₂F₆ evidently produces a polymer withoutthis affinity for tungsten silicide, thereby eliminating micromasking.

The inventive process includes a high pressure atmosphere in order toproduce a faster oxide etch rate. High pressure results in a higherfluorine radical flux on the oxide surface. When combined with high rfpower, the etch rate is increased. High pressure and rf power do havedrawbacks, however. Although rf induced ion bombardment assists in oxideetch, it also contributes to photoresist erosion, which is undesirable.Further, if rf power is too high, the resist will “burn” or reticulate.Higher pressure makes a thicker atmosphere, scattering ions and gasradicals in the plasma, resulting in more sidewall etching than with alow pressure system.

The oxide etch step of the inventive method includes an overetch ofabout 45 seconds to fully clear all residual oxide. Although theC₂F₆/CF₄/CHF₃ gas mixture etches underlying polycide during overetch,the etch continues to be anisotropic because of the sidewall passivationprovided by the halocarbon-derived polymer and from the carbonintroduced by eroding resist. After oxide has been cleared, thepolycide-to-resist etch rate ratio is approximately 1.8:1.

Polycide etch step details are now provided. Although preferredparameter values have been stated, plasma power density can range withinabout 0.18-2.0 W/cm², the gap can vary within about 0.5-2.5 cm, 0.8-1.5cm being the preferred range, and the pressure can range within about0.200-0.550 torr, although about 0.300-0.425 torr is preferred.Quantities of the gases may vary, as long as at least about 50 sccm Heis provided. It is contemplated that SiCl₄ or BCl₃ or a combinationthereof might be used to provide additional Cl₂, if desired.

The lower pressure of the polycide etch allows for more ion bombardment,which, with resist erosion and the Cl₂ concentration, determines theetch rate and profile of tile silicide and poly layers 12 and 13. Cl₂provides the necessary selectivity to the polycide, so that minimalunderlying gate oxide 14 is etched. Fluorine can also be used, but Cl₂is preferred because it provides superior selectivity. The resist usedmust therefore be able to reasonably withstand a chlorine-based plasma.The preferred embodiment utilizes Hunt's 6512 resist, developed withHunt's photoresist developer 428. It is realized that other resists,developers, and mask layer compositions can be used as well.

An additional benefit of the inventive method is the ability to usecarbon generated by the resist to help passivate polycide sidewalls,which means that carbon-containing gases do not have to be added to thegas mixture during polycide etch.

There is an upper rf power limit that can be safely used before thepoly-to-gate oxide selectivity is reduced to the point where the polycannot be completely etched without removing all of the exposed gateoxide. The inventive process provides a selectivity of approximately13:1. Variations in the chlorine flow and total pressure do notsignificantly change this selectivity, although an increase in rf powerreduces it.

In both of the inventive steps, helium is added to improve etchuniformity. The pressure, power, and various gas quantities are balancedto produce the fastest etch rates while preserving selectivity.

Clearly, in view of the above disclosure, other embodiments of thisinvention will present themselves to those of ordinary skill insemiconductor processing, such as applying the invention to other kindsof masking layers, oxide, silicide, such as tungsten silicide, tantalumsilicide, molybdenum silicide, and titanium silicide, and poly, andvarying thickness and doping of each layer etched. Since the inventiveprocess includes one step for polycide etch, a simple oxide/polystructure can be etched instead of an oxide/silicide/poly structure,without materially altering the process. It is also conceivable thatplasma power density and gap may be varied, gas quantities adjusted,similar gases substituted, or some other inert material used for thecathode, to achieve the same or similar results. Gas quantities may alsobe changed while preserving essentially similar ratios of one gas toanother. Another make of reactor might also be chosen. These variationsand others are intended to be circumscribed by these claims.

What is claimed is:
 1. A method to anisotropically etch at least onelayer in a group of layers consisting of a silicide layer, apolycrystalline silicon layer, and a polycide layer located over a layerof gate oxide on a substrate, said anisotropic etching producing aprofile at or near 90° from horizontal in a parallel plate plasma etchreactor having a first electrode and an inert second electrode comprisedof anodized aluminum, said method comprising: providing a gap ofapproximately 1.0 cm between said first electrode and said inert secondelectrode; mounting said substrate on said first electrode; providing aplasma atmosphere within said parallel plate plasma etch reactor, saidplasma atmosphere including a pre sure of approximately 0.325 torr, Cl₂at a rate of approximately 90 sccm, H at a rate of approximately 70sccm, and a plasma power density of approximately 0.57 W/cm²; andanisotropically etching said at leas one layer in said group of layersconsisting of a silicide layer, a polycrystalline silicon layer, and apolycide layer located over said layer of gate oxide.
 2. A method toanisotropically etch at least one layer in a group of layers consistingof a silicide layer a polycrystalline silicon layer, and a polycidelayer located over a layer of gate oxide on a substrate, saidanisotropical etch producing a profile at or near 90° from horizontalwith respect to said substrate in a parallel plate plasma etch reactorhaving a first electrode and an inert second electrode comprised ofanodized aluminum, said first electrode and said inert second electrodehaving gap therebetween, said method comprising: providing a gap withinthe range of approximately 0.5 cm to 2.5 cm; mounting said substrate onsaid first electrode; and providing a plasma atmosphere within saidparallel plate plasma etch reactor including Cl₂ and He, a pressurewithin the range of approximately 0.200 to 0.550 torr, and a plasmapower density within the range of approximately 0.18 to 2.0 W/cm²; andanisotropically etching said at leas one layer in said group of layersconsisting of said silicide layer, said polycrystalline silicon layer,and said polycide layer located over said layer of gate oxide.
 3. Amethod to anisotropically etch at least one layer in a group of layersconsisting of a silicide layer, a polycrystalline silicon layer, and apolycide layer located on a portion of a substrate, said anisotropicaletch producing a profile at or near 90° from horizontal with respect tosaid substrate using a parallel plate plasma etch reactor including afirst electrode and an inert second electrode comprised of anodizedaluminum, said first electrode and said inert second electrode having agap therebetween, said method comprising: providing a gap within therange of approximately 0.8 cm to 1.5 cm; mounting said substrate on saidfirst electrode; and providing a plasma atmosphere w thin said parallelplate plasma etch reactor including Cl₂ and He, a pressure within therange of approximately 0.300 torr to 0.425 torr, and a plasma powerdensity within the range of approximately 0.18 W/cm² to 2.0 W/cm² toanisotropically etch said a least one layer in said group of layerscomprising said silicide layer, said polycrystalline silicon layer, andsaid polycide layer located on said portion of said substrate.
 4. Amethod to anisotropically etch a structure in situ to produce a profileat or near 90° from horizontal with respect to said structure, saidstructure including a first layer of an oxide of silicon on a secondlayer selected from the group consisting of silicide, polycrystallinesilicon, and polycide, said structure located on a substrate, saidanisotropic etch comprising using a parallel plate plasma etch reactorincluding a first electrode and an inert second electrode comprised ofanodized a aluminum, said first electrode and said inert secondelectrode having a gap therebetween, said method comprising: placingsaid substrate on said first electrode; providing a first high pressureat sphere within said parallel plate plasma etch reactor, said firsthigh pressure atmosphere including C₂F₆, CHF₃, CF₄, and He; exposingsaid first layer to a first plasma having a first high power density toexpose at least a portion of said second lay overetching said firstlayer to substantially remove said oxide of silicon; providing a secondhigh pressure atmosphere within said parallel plate plasma etch reactorincluding Cl₂ and He; and exposing said second layer to a second plasmahaving a second high power density.
 5. The method of claim 4, wherein:said first high pressure atmosphere includes a pressure of approximately2.3 torr, C₂F₆ at a rate of approximately 50 sccm, CHF₃ at a rate ofapproximately 32 sccm, CF₄ at a rate of approximately 40 sccm, a d He ata rate of approximately 100 sccm; said first plasma includes a firsthigh power density of approximately 1.9W/cm²; a gap of approximately0.48 cm exists between said first electrode and said inert secondelectrode for said first high power density; said second high pressureatmosphere includes a pressure of approximately 0.325 torr, Cl₂ at arate of approximately 90 s cm and He at a rate of approximately 70 sccm;said second plasma includes a second high power density of approximately0.57 W/cm²; and a gap of approximately 1.0 cm exists between said firstelectrode and said inert second electrode for said second high powerdensity.
 6. The method of claim 4, wherein: said first high pressureatmosphere includes a pressure within a range of approximately 1.8 torrto 3.0 torr; said first plasma includes a first hi h power densitywithin a range of approximately 0.18 to 4.0 W/cm²; a gap within a rangeof approximately 0.3 to 0.6 cm exists between said first electrode andsaid inert second electrode for aid first high power density; saidsecond high pressure atmosphere includes a pressure within a range ofapproximately 0.200 torr to 0.550 torr; said second plasma includes asecond high power density within a range of approximately 0.18 to 2.0W/cm²; and a gap within approximately 0.5 to 2.5 cm exists between saidfirst electrode and said inert second electrode for said second highpower density.
 7. The method of claim 4, wherein: said first highpressure atmosphere includes a pressure within a range of approximately2.2 torr to 2.4 torr; said first plasma includes a first high powerdensity within a range of approximately 0.18 to 4.0 W/cm²; a gap withinapproximately 0.38 t 0.52 cm exists between said first electrode andsaid inert second electrode for said first high power density; saidsecond high pressure atmosphere includes a pressure within a range ofapproximately 0.300 torr to 0.425 torr; said second plasma includes asecond high power density within a range of approximately 0.18 to 2.0W/cm²; and a gap within approximately 0.8 t 1.5 cm exists between saidfirst electrode and said inert second electrode for said second highpower density.
 8. The method of claim 4, wherein: said first highpressure atmosphere includes more C₂F₆ than CF₄ and more CF₄ than CHF₃;said first high pressure atmosphere includes at least approximately 5sccm He; and said second high pressure atmosphere includes He at a rateof at least approximately 50 sccm.
 9. The method of claim 4, wherein thestructure includes a mask layer that releases carbon when subjected to aplasma and resists chlorine.